The overall goal of this proposal it to develop structure-based approaches to discover new G protein coupled receptor (GPCR) ligands having new signaling properties and specificities. GPCRs are involved in regulating virtually every aspect of physiology and are pivotal targets for drug discovery. Until now, ligand discovery efforts for GPCR has been empirically driven, and though this has had successes, it has restricted the field to sites precedented by canonical, often natural ligands. Considering the remarkable progress in identifying new GPCRs over the past two decades, drug discovery for this family of receptors using classical approaches has been disappointing. Most available ligands act at orthosteric sites, competing directly with the natural hormones and neurotransmitters. In the rare circumstances that they bind allosterically, their discovery has been fortuitous, their optimization difficult, as has been the dissection of their signaling. The recent efflorescence of GPCR X-ray structures was followed by the application ligand docking methods demonstrating the feasibility of this approach for the discovery of novel orthosteric ligand chemotypes for several GPCRs. We propose an integrated program of structure-based exploitation of GPCRs for new ligand chemotypes with an emphasis on allosteric ligands, their testing for new signaling properties, the determination of their structures bound to their GPCRs, and their optimization for affinity and signaling. This proposal builds on a network of existing collaborations among the labs of Kobilka, Shoichet, Sunahara and Gmeiner over the past four years. These four investigators bring together a unique combination of expertise in GPCR structural biology, ligand docking, GPCR pharmacology and function, and medicinal chemistry. Preliminary studies from this group demonstrate the feasibility and potential value of this approach.